Ferritic stainless steel sheets and steel pipes have been frequently used in components of exhaust systems. For example, SUH409L is a steel that contains 11% of Cr, in which C and N are fixed by Ti so as to prevent sensitization of welded portions and to attain excellent workability. SUH409 has sufficient high-temperature characteristics at 700° C. or lower, and in addition, SUH409 exhibits a certain degree of resistance to condensed water corrosion; and therefore, SUH409L is most frequently used. In addition, steels are also used which have enhanced resistance to condensed water corrosion and resistance to salt corrosion, such as AISI439 which contains 17% of Cr and in which C and N are fixed by Ti, SUS436J1L and SUS436L which further contain Mo, and the like.
Meanwhile, due to diversification of fuels such as bio-fuels and the like, or regulations for improving gas mileages in recent years, corrosion environments for materials for an automobile exhaust system are changing. In addition, in emerging markets, a decrease in the pH of exhaust gas-condensed water generated from poor fuels is becoming a problem. In consideration of such situations, it has come to be considered that a higher level of corrosion resistance is required. In response to the above, SUS436L and the like which contain Mo so as to enhance the corrosion resistance are regarded as being appropriate for materials for exhaust systems in the related art. However, in a situation of the current steep rise in resource prices, Mo is known as one of the most expensive alloying elements, and there has been a long desire for a new type of steel which contains no Mo or contains an amount of Mo as low as possible, and exhibits corrosion resistance similar to or superior to that of SUS436.
Regarding the above problems, several techniques have been proposed in the related art.
For example, Patent Document 1 discloses a steel that contains both of Cu: 0.3% to 2.0% and P: 0.06% to 0.5% instead of containing Mo so as to secure corrosion resistance similar to or superior to that of a 17Cr-1Mo steel. However, since both of Cu and P are solid solution strengthening elements, deterioration of the workability is inevitably caused when a large amount of Cu and P are included. Workability as well as corrosion resistance is also an indispensable property for materials that are applied to components in exhaust systems; and therefore, it is difficult to apply the above-described steel to the components in exhaust systems.
Patent Document 2 discloses a steel that contains both of Cu: 0.5% to 2.0% and V: 0.05% to 2.0% instead of containing Mo so as to secure corrosion resistance similar to or superior to that of a 17Cr-0.5Mo steel. However, similarly to the case of Patent Document 1, since Cu is a solid solution strengthening element, deterioration of the workability is inevitably caused when a large amount of Cu is included. In addition, similarly to Mo, V has a problem of being an expensive alloying element.
Patent Document 3 discloses a steel in which the amount of Si is reduced in order to secure workability, and 0.01% to 1.0% of Co is included in order to improve the corrosion resistance without impairing the workability, and in the steel, corrosion resistance similar to that of 18Cr—Mo steel is secured. However, a small content, approximately 0.05%, of Co is sufficient only in the case where approximately 25% of Cr is included. The content of Co needs to be approximately 0.5% in the case where approximately 18% of Cr is included. In addition, similarly to Mo, Co also has a problem of being an expensive and rare alloying element.
Patent Document 4 discloses a steel in which either one or both of Ni: 0.1% to 2.0% and Cu: 0.1% to 1.0% are included at a total amount of 0.6% or more so as to enhance the corrosion resistance without including Mo. However, in order to obtain corrosion resistance superior to that of SUS436L, it is necessary to include large amounts of alloying elements, such as a steel containing 20% of Cr and 1% of Ni. Therefore, there is a problem in that the above-described technique does not necessarily reduce the costs. In addition, Cu is an element that strengthens a steel more than Mo, and there is a problem in that the workability deteriorates even at a small content of Cu.
Meanwhile, as an interesting technique that is approximately consistent with the purport of the present invention, which is a lean alloy (composition having low contents of alloying elements), a technique has been disclosed in which a steel contains extremely small amounts of Sn and Sb, which are alloying elements and gained little attention in the related art, so as to improve the characteristics of the steel.
For example, Patent Document 5 discloses a ferritic stainless steel which contains 0.02% to 0.2% of Sb so as to improve the oxidation resistance. Patent Document 6 discloses a ferritic stainless steel sheet which contains either one or both of Sn and Sb at a content of 0.005% to 0.10% so as to prevent intergranular corrosion of P. Thereby, there does not occur surface scratches which are caused by intergranular corrosion when the steel sheet is pickled using sulfuric acid. In addition, Patent Document 7 discloses that it is effective to include 0.5% or less of Sn for suppressing intergranular corrosion that is caused by Cr carbonitrides in welded heat-affected zones.
However, in the above-described techniques, there is no description regarding the resistance to salt corrosion and the resistance to condensed water corrosion after heating of components in exhaust systems, which will be dealt with in the present invention.
Meanwhile, in recent years, attention has been paid to an effect of Sn and Sb for improving corrosion resistance so as to develop a new type of steel.
For example, Patent Document 8 discloses a ferritic stainless steel sheet that contains either one or both of Sn and Sb, and is excellent in crevice corrosion resistance. In addition, Patent Document 9 also discloses a ferritic stainless steel that contains Sn and Sb as selective elements in order to suppress flow of rusting from crevice portions.
All of the above-described techniques deal with crevice corrosion. In the ferritic stainless steel, it is necessary to include proper contents of alloying elements in order to suppress the crevice corrosion. Therefore, in these techniques, the contents of the alloying elements are generally large; and thereby, characteristics other than the corrosion resistance (for example, workability and costs) do not necessarily fulfill the satisfactory levels. Therefore, there is a possibility of better optimization.